Ensatina's basic story was laid out by Robert Stebbins 30 years before Tom was
born in 1977. Based on the ring-like distribution of the different forms, Robert
had proposed that the species started off in Northern California and Oregon and
then spread south along both sides of the Central Valley, which was too dry and
hot for salamanders.1

According to Robert's hypothesis,
as the pioneering populations moved south, they evolved into several subspecies
with new color patterns and adaptations for
living in different environments. By the time they met again in Southern California
as the subspecies eschscholtzii and klauberi, he argued, they had
each evolved so much that they no longer interbred  even though the subspecies
blended into one another around the rest of the ring. Since species are often
defined by their inability to interbreed with other species, Ensatina seemed
to represent the whole process of speciation  all the gradual changes that
accumulate in two lineages and that wind up making them incompatible with one
another.

Of course, since this all would have happened millions of years ago, Robert
wasn't around to observe any of it. He based his ideas on the morphology,
or body form, of the subspecies  in this case, their color patterns.
First, neighboring subspecies were more similar to one another than to those
across the ring and seemed to blend into one another. From this, he hypothesized
that Ensatina represented a ring species. Robert also noticed that
the northern coastal form, called picta, had a pattern of colors that
seemed to encompass the other subspecies. It was easy to imagine how the more
specialized southern forms could have evolved from picta. Based on this,
Robert hypothesized that the two southward-moving Ensatina lineages
had both emerged from picta's immediate ancestors.